Let a nested behavior without a start node run from On start (#230)

A behavior built as a plain behavior, with On start and no Behavior
start, did nothing when another behavior called it through Run
behavior. The node only showed a warning. Such a behavior should run
as a call too, without the author having to rebuild its entry.

- New callEntries() in nested.ts returns the Behavior start node or,
  when there is none, every On start node. callProblem uses it too.
- GraphInterpreter.call() follows all entries one after the other,
  in graph order, in the same frame and with the same variables.
- Behavior end still returns what reaches it. A call through On start
  has no inputs, because only the start node declares them.
- It is a fallback only. With a Behavior start present, On start
  nodes stay out of the call.
- The node warning now reads "No start or On start node - does
  nothing" and only shows when neither exists.
- Keep and local state machines behave as before.

Tests:
- ./check.sh: 163 files, 2396 tests, CHECK EXIT 0 (before 2392).
- 4 new unit tests in tests/core/nested-behavior.test.ts: the flow
  after On start runs, the return value arrives, several On start
  nodes run in graph order, a start node keeps On start out.
- e2e 19: ALL PASSED, 35 checks. Only the expected warning text
  changed, the check sum stays at 1385.
- Counter-checks: without the fallback 3 tests fail. With only the
  first On start exactly the order test fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/nested-behavior.test.ts

@@ -198,7 +198,7 @@ describe('a call that cannot do anything says so', () => {
     call.params[BEHAVIOR_PARAM] = 'gone';
     expect(problem()).toBe('That behavior is gone');
     call.params[BEHAVIOR_PARAM] = makeBehavior(fixture, 'Empty').id;
-    expect(problem()).toBe('No start node – does nothing');
+    expect(problem()).toBe('No start or On start node – does nothing');
     call.params[BEHAVIOR_PARAM] = callee.id;
     expect(problem()).toBe(null);
     // Only a call can be dead this way; no other node type may be marked.
@@ -245,6 +245,78 @@ describe('a call that cannot do anything says so', () => {
   });
 });
 
+describe('a called behavior without a start node begins at On start (#230)', () => {
+  /** A caller whose On start runs `callee` and logs "after" once the call is done. */
+  function logCaller(fixture: ProjectFixture, callee: BehaviorAsset) {
+    const caller = makeBehavior(fixture, 'Caller');
+    const edit = graphEditor(fixture, caller);
+    const onStart = edit.addNode('OnStart');
+    const call = edit.addNode(RUN_BEHAVIOR, { [BEHAVIOR_PARAM]: callee.id });
+    edit.wire([
+      [onStart, 'exec', call, 'exec'],
+      [call, 'exec', edit.addNode('Log', { message: 'after' }), 'exec'],
+    ]);
+    return { caller, call };
+  }
+
+  it('runs the flow behind its On start, and the call is not marked dead', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Plain');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'sub' }), 'exec');
+    const { caller, call } = logCaller(fixture, callee);
+
+    expect(callProblem(call, graphLookup(fixture.project), caller.id)).toBe(null);
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['sub', 'after']);
+  });
+
+  it('still hands back what its end node was given', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Seven');
+    const cEdit = graphEditor(fixture, callee);
+    const onStart = cEdit.addNode('OnStart');
+    const end = cEdit.addNode(BEHAVIOR_END);
+    fixture.stack.execute(new AddPortCommand(callee.id, end.id, { key: 'out1', kind: 'number' }));
+    cEdit.wire([
+      [onStart, 'exec', end, 'exec'],
+      [cEdit.addNode('Number', { value: 7 }), 'value', end, 'out1'],
+    ]);
+    const caller = makeBehavior(fixture, 'Caller', ['result']);
+    const edit = graphEditor(fixture, caller);
+    const call = edit.addNode(RUN_BEHAVIOR, { [BEHAVIOR_PARAM]: callee.id });
+    const set = edit.variables(['result'], { write: true });
+    edit.wire([
+      [edit.addNode('OnStart'), 'exec', call, 'exec'],
+      [call, 'exec', set, 'exec'],
+      [call, 'out1', set, 'result'],
+    ]);
+
+    expect(runOnStart(fixture, caller, { result: 0 }).ctx.variables.result).toBe(7);
+  });
+
+  it('runs every On start, in graph order, as the behavior would on its own', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Twice');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'one' }), 'exec');
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'two' }), 'exec');
+    const { caller } = logCaller(fixture, callee);
+
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['one', 'two', 'after']);
+  });
+
+  it('is only a fallback: with a start node, its On start stays out of the call', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Both');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode(BEHAVIOR_START), 'exec', cEdit.addNode('Log', { message: 'start' }), 'exec');
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'on start' }), 'exec');
+    const { caller } = logCaller(fixture, callee);
+
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['start', 'after']);
+  });
+});
+
 describe('declared ports', () => {
   it('renaming one rewrites the edges in this graph and in every calling one', () => {
     const fixture = makeProject();